Patent GB-555054-A (GB)
Title: Improvements in or relating to the consolidation of joining surfaces of organic thermoplastic material
IPC: B29C65/50
Filed: 1941-08-18
Abstract: 555,054. Seaming non-metallic sheet material. JARRARD, W. J., MORITZ, M. R., and ZADE, H. P. Aug. 18, 1941, Nos. 10506, 10658, 11008, 11009 and 11010. [Class 42 (i)] [Also in Group XI] Two joining faces of a thermoplastic or thermosetting material are consolidated by the employment of conducting electrodes which set up an alternating electric field of high frequency at the joining faces so that such faces are sufficiently softened by heat to enable the consolidation to be effected. Thermoplastic materials treated included cellulose ethers and esters, polymers of acrylic acid esters and alkyl acrylic esters, vinyl resins, polyalkylenes, polystyrenes. Thermosetting materials treated include those which can be softened or rendered adhesive by heat. The arrangement of the electrodes is preferably such that the equipotentials are parallel to and are crowded together near the surface to be heated. In joining two surfaces of thermoplastic or thermosetting material, the surfaces may be pressed together by pressure applied to the electrodes. The electrodes are chosen of such shape chat the desired distribution and volume density of the equipotentials is obtained. In an example, two sheets of resin derived from polyvinyl chloride are placed, with edges overlapping between two flat electrode bars which are pressed together. The bars are connected to an alternating source of frequency about 107 cycles per second whereby the joining surfaces are softened, pressed together and consolidated. In another example two plates of methyl methacrylate have their edges upturned and pressed together between electrodes to which an alternating field is applied. When it is required to unite a surface of thermoplastic material to a surface of thermosetting material such as a phenol formaldehyde resin or ebonite one or both surfaces may be treated with a solvent or plasticizer. Movable electrodes may be employed, e.g. in the case of a joint drawn between one or more pairs of rollers. Other materials whose dielectric loss factors are higher than the plastics may be inserted beween the joining surfaces so that heat is generated in them If the specific inductive capacity of the inserted material is lower than that of the plastic, the equiootentials will crowd into it thereby increasing the heating. The material is not used as an adhesive. Electric conducting materials may also be inserted in the joint to provide an equipotential surface. In some cases two metal electrodes cannot produce the desired equipotential distribution, and one or more additional electrodes which may be at earth potential or a biassed potential may be employed to overcome inequalities of heating or to localise the heating. Another example is given of the formation of a butt joint between two plates of thermoplastic material-polyvinyl chloride. Four electrodes are used near the joint, two interconnected engaging one of the plates and the other two similarly engaging the other plate If the electrodes are in air the equipotentials which pass through the plastic bend away from the joint. and to ensure their concentration near the joint each electrode is embedded in a dielectric such as that known under the Registered Trade Mark " Mycalex." In some cases with a single plastic or with composite bodies which are unsymmetrical, uniform heating may be ensured by varying the distribution of the field in a periodic manner, e.g. by using two or more interdependent systems of electrodes with potentials varying by 90 degrees or a multiple thereof, ensured oy coupling them througn capacities; or by coupling the electrode systems to separate generators interconnected to ensure the desired phase difference; or by moving the work to the electrodes relatively to one another; or by moving pieces of dielectric in the field; or by periodically connecting and disconnecting some of the electrodes. Specification 555,196, [Groucp XXXVI], is referred to.